Aluminum and aluminum-silicon alloys are typically used as the interconnect materials for semiconductor devices. The aluminum or the aluminum-silicon alloy is deposited over a semiconductor wafer during the latter stages of the device fabrication process. Then, a photoresist is applied, exposed, and developed to define the metallization pattern. Using the resist as a mask, the exposed portions of the aluminum or aluminum-silicon alloy are removed using a wet etching process. The photoresist pattern is removed and the desired metallization pattern is formed.
In the wet etching process, the semiconductor wafer with the metallization layer is contacted with an acid etching solution, such as an aqueous solution of phosphoric (H.sub.3 PO.sub.4) and nitric (HNO.sub.3) acids. However, it has been found that after a period of time the etch rate of the metallization pattern decreases. As the etch rate decreases, the wafers are etched in an uneven manner whereby portions of the metallization pattern are underetched or overetched.
It was also observed that a precipitate builds up within the etch tank when the etch rate decreased. This indicates that the nitric acid concentration has become depleted. The nitric acid concentration is reduced because the acid is consumed during the etching of the aluminum or aluminum-silicon metallization also, some of the nitric acid depletion can be attributed to evaporation. Previously, when the etch rate reached unacceptable levels, the entire etch bath was disposed of and replaced with a fresh etching solution. It would be desirable to replenish the depleted nitric acid so that the downtime of the etching operation is reduced and so that the wafer is etched in a substantially uniform manner throughout the life of the bath.
It is known in the etching art to monitor the etching solution so that the solution can be continuously regenerated. For example, in U.S. Pat. No. 4,058,431 to R. Haas, entitled "Method of Etching Copper and Copper Alloys," issued Nov. 15, 1977, a method is disclosed for using both pH and specific gravity measurements to monitor the etching solution. In response to these measurements, the appropriate components of the etching solution are added so as to continuously regenerate the etching solution. However, it has been found that pH measurements are not sensitive enough to detect the subtle compositional changes in the H.sub.3 PO.sub.4 -HNO.sub.3 etching solution used to etch metallization patterns on semiconductor wafers.